Online print services give reasonably detailed info. your artwork contains text, you have made a file that can't be used without having your fonts installed (=you didn't outline the texts to curves).the printer wants a print PDF, you give to them one made for onscreen watching.the printer wants uncolored places as transparent, you give them a file with no transparency.not noticed that CMYK colors need a white background.too complex vector drawing, impossible to plot with the plotter in reasonable time.too big bitmap resolution, impossibly high file size.basic wrong data type (the printer wants a bitmap, you have a vector file or reversed).

The most important thing to do soon is to get a detailed info 'How the printer wants the final file to be prepared' Without that knowledge all your efforts can vapourize just because your work is not printable.

after completing your artwork: the final output file parameters due the printing system you want to use.

work color space (assuming you have color management in use, a calibrated monitor, you want to see onscreen a plausible forecast of the printed result and you have got the color profile of the printing system).

raster effects rendering resolution, 300 DPI should be generous for printing on shirts.

the physical print size of the artboard (= to be able to give to some elements exact wanted sizes).

the color model (RGB, CMYK, BW, idexed ).

You must have done some investigations on what is possible in this media (size, available colors, only short time visible, maybe partially covered, competes for attention)

Hopefully you have a clear idea on what to say and by which means you can say it.

Design t shirt adobe illustrator